1. A new fluorometric assay for phospholipase activity has been used. It was found that bovine serum albumin activates and inhibits phospholipases from different sources. The optimal conditions for the assay have been investigated. The mechanism of the serum albumin effects have also been studied. Albumin may interact with the enzyme, or with the substrate, to modulate the rate of liposome lysis by phospholipases. 2. Energy transfer studies on human serum albumin have been performed to estimate intramolecular distances between various binding sites on the protein. 3. Stopped-flow fluroescence measurements on the rate of labeling of proteins show that dyes attach in the millisecond range. The rate of acetylation of proteins was also studied with fluorogenic reagents.